This applicant's pending application Ser. No. 15,587, filed Feb. 27, 1979, discloses a DC arc furnace hearth formed by a layer of carbon bricks above which an electrically relatively non-conductive refractory compound is rammed to contain a melt, and metal bars or rods inserted through the compound so that their upper ends are exposed to the melt and their bottom ends are in electrical connection with the bricks, forming an electrically conductive hearth. An arcing electrode above the melt can be connected to one pole of DC power and the hearth connected to the other so that an arc is formed between the electrode and melt.
The rammed compound can be of the usual type which while moist or soft is rammed onto the bricks of the conventional electric arc furnace hearth. While it is still soft, the metal bars or rods can be driven down through this compound so that the bars' upper ends are exposed to the melt and their bottom ends are in electrical connection with the electrically conductive bricks. Preferably a layer of electrically conductive soft material, such as a graphite layer, is first layed on the bricks so this layer is beneath the rammed compound for penetration by the rods and the establishment of a better electrical connection. For a power connection a metal plate is positioned below and in electrical contact with the conductive bricks.
The above installation technique is complicated, runs the risk that the rods, which should be arranged as a cluster of uniformly interspaced bars, may be inaccurately positioned, and in general, the making of such a furnace hearth, although entirely practical, can be troublesome.
Furthermore, before the furnace can be started up, the moist rammed non-conductive refractory must be dried and fired just as is the case when lining or relining the usual hearth of any electric arc furnace.
The hearth described has the practical advantages that it provides an electrically conductive hearth forming a hearth connection for carrying the DC current into the melt as required to form the arc with the usual arcing electrode connected with opposite polarity to the power source, normally so that the arcing electrode operates cathodically. This eliminates the need for the usual side positioned melt electrode or connection with its attendant disadvantages and may eliminate the need for the usual starting electrode conventionally required to melt down scrap to a degree making the conventional hearth electrode effective during start-up of the furnace.
When the furnace is in operation, the upper ends of the metal bars exposed at the top level of the rammed refractory and in contact with the melt, themselves melt downwardly through the rammed compound to a limited extent, but since the bars can be made of the same metal as the melt in the hearth, usually steel, the melt does not necessarily become appreciably altered in its composition.
The object of the present invention is to enjoy the advantages of the described kind of electrically conductive hearth while avoiding its installation disadvantages.